The present invention generally relates to document reading apparatuses, and more particularly to a document reading apparatus which uses a contact type image sensor.
Conventionally, there are document reading apparatuses which use a contact type image sensor. FIG. 1 shows a typical example of such conventional document reading apparatuses.
The conventional document reading apparatus shown in FIG. 1 has a contact type image sensor 5 which includes a transparent protection (or contact) glass 1, a light emitting diode (LED) array 2 having an array of LEDs, a rod lens array 3 having an array of rod lenses, a photoelectric conversion element array 4 having an array of photoelectric conversion elements and the like. A white guide plate 8 confronts a surface of the protection glass 1 of the contact type image sensor 5. A predetermined gap is formed between the white guide plate 8 and the surface of the protection glass 1. Document transport roller pairs 9 and 10 are respectively provided on both sides of the contact type image sensor 5. For example, a document 7 is transported between the white guide plate 8 and the protection glass 1 by the document transport roller pair 9 and is read by the contact type image sensor 5. After the document 7 is read, the document 7 is ejected by the document transport roller pair 10.
FIG. 2 shows a conceivable document reading apparatus. In FIG. 2, those parts which are essentially the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. In the document reading apparatus shown in FIG. 2, a rotatable platen roller 6 makes contact with the protection glass 1 at a document reading position "A" of the contact type image sensor 5. An outer peripheral portion of the platen roller 5 is made of a white resilient material. The document 7 is read when the document 7 is transported in a state pinched between the protection glass 1 and the platen roller 6.
However, the document reading apparatus shown in FIG. 2 suffers problems in that the outer peripheral surface of the platen roller 6 easily becomes dirty due to the contact with the document 7, and it is difficult to completely remove the dirt on the platen roller 6 especially because the outer peripheral portion of the platen roller 6 is made of a resilient material such as rubber. In other words, when the outer peripheral portion of the platen roller 6 is made of rubber, for example, very fine dirt adheres on the rubber surface which actually has minute irregularities and such dirt often cannot be removed by an alcohol remover (or cleaner) or the like. For this reason, the white surface of the platen roller 6 cannot be maintained perfectly white because of the dirt which is difficult to remove, and as a result, a shading correction of the contact type image sensor 5 cannot be carried out accurately and the reading accuracy becomes poor. In addition, since the platen roller 6 is pushed against the surface of the protection glass 1 until the document 7 reaches the document reading position "A", a large load acts on the platen roller 6 when rotating the platen roller 6.
On the other hand, according to the document reading apparatus shown in FIG. 1, the white guide plate 8 does not easily become dirty like the platen roller 6 of FIG. 2 because the document 7 is transported within the gap formed between the white guide plate 8 and the protection glass 1. However, there are problems in that the document 7 easily floats from the surface of the protection glass 1 due to the gap and an out-of-focus state is generated thereby. The out-of-focus state is easily generated by a slight floating of the document 7 because a focal distance of the optical system of the contact type image sensor 5 is short. This out-of-focus state is notable especially when the document 7 is a thick and stiff paper, includes folds and the like. In order to prevent the out-of-focus state, it is conceivable to reduce the gap between the white guide plate 8 and the protection glass 1 or to make the white guide plate 8 push against the surface of the protection glass 1 by the weight of the white guide plate 8 itself. However, a satisfactory document transport cannot be achieved when either of such measures is taken especially when the document 7 is a thin paper such as an onion skin paper, and a paper jam is likely to occur. Furthermore, when the white guide plate 8 becomes dirty due to the contact with the document 7, it is difficult to carry out an accurate shading correction of the contact type image sensor 5 and the reading accuracy becomes poor.